What am I missing?

[Hammermechanicman]

Bad boys i have worked on have a 12ga black wire that grounds everthing. If he follows it back in the harness it will probably connect to a half dozen or so smaller black wires. 2 clues. He said with key off 0 volts, key on test light from wire to ground very dim. That indicates he is connecting through a load. Second, he said if he grounds the wire everthing works fine. The black wire needs to be grounded.